Method of making an electroacoustic transducer

ABSTRACT

The invention provides a low cost electroacoustic transducer. The transducer housing has incorporated into its rear surface, a counterbored or cavity section opposing a flat portion on the outside surface of the housing. This construction results in a clamped vibratile disk to which a piezoelectric ceramic disk is attached. In addition, the invention includes a low cost method for adjusting the resonant frequency. First, the resonant frequency of the structure is measured after assembly. Then, material is removed from the external flat surface of the housing to achieve the desired resonant frequency.

United States Patent 1191 Barrow [75] Inventor:

Y [73] Assigne e: Massa Division, Dynamics Corpora [54] METHOD OF MAKING AN ELECTROACOUSTIC TRANSDUCER Gilbert C. Barrow, Hingham, Mass. Y

tion of America, Hingham, Mass.

[22] Filed: Mar. 18, 1971 21 Appl. No.: 125,880

Related u.s. Application Data [63] Continuation of Ser. No.' 866,785, Sept, 16, 1969,

abandoned.

52] 115.01. ..29/25.35,29/593,29/594,' y 5 179/110 A, 179/181 R, 310/82 5 Int. 'CI ..B0 lj l7/00,H04rl7/00 '[58'] Field 61 Search ..-.....-.-.....29/25.35, 594, 593;

3lO/8.2,8.5; 179/110 A, 18] R; l8l/32'R 156] References Cited UNITED STATES PATENTS 3,128,532 t/1 964 Massa ..29/25.35

1 1 June5, 1973 3,307,052 2 1967 Neilson 61111 ..310/8 3,698,051 10/1972 Miller 29 2535 3,670,406' 6/1972 Weber ..29 593 2,639,392 5 1953 Warner,.lr ..29/593 UX 2,905,260 9/1959 Williams ..181 32R 3,578,995 5 1971 Massa ..310/82 Primary ExaminerCharles W, Lanham Assistant ExaminerCarl E. Hall AltOrney- Louis Bernat 57 ABSTRACT The invention provides a low cost electroacoustic transducer. The transducer housing has incorporated into its rear surface, a counterbored or cavity section opposing a flat portion on the outside surface of the housing. This construction results in a clamped vibratile disk to which a piezoelectric ceramic disk is attached. In addition, the invention includes a low cost method for adjusting the resonant frequency. First, the resonant frequency of the structure is measured after assembly. Then, material is removed from the external flat surface of the housing to achieve the desired resonant frequency.

9 Claims, 3 Drawing Figures ers and more'particularly to very low cost transducers especially although not exclusively well suitedfor use in ultrasonic burglar alarm systems.

Burglar alarms are here cited primarily because they are exemplary of many modern systems designed to utilize ultrasonic devices of the type described herein. These devices have a wide spread, popular appeal if they can be mass produced with high quality precision Y and at very low cost. These'devices should also be, and remain through out their life, highly reliable in operation. Also, these devices should be sufficiently attractive in appearance so that they may be used with little or no added escutcheons, embellishments, or the like.

Sonically, the transducers are especially well adapted for operation in a predetermined frequency band. Insofar as assembly is concerned, thisparticular frequency band may be adjusted for any given transducer even after it has been completely manufactured.

I do not suggest any particular frequency band as greatly tobe preferred over any other frequency band. However, I do believe that the transducer may find an economic value when used in either the higher audible frequencies or the ultrasonic frequencies, (e.g. 10,000 cps and higher). Among other things, this is because more conventional forms of transducer devices are very costly when designed to operate at frequencies which are higher than approximately 10,000 cycles per second.

Accordingly, an object of this invention is to provide new and improved electroacoustic transducers which may be mass produced at relatively low cost and yet provide high quality and reliable operation.

Another object of the invention is to provide electroacoustic transducers which operate efficiently in a desired frequency band. Here an object is to provide simple and easily adjusted means for controlling the resonant frequency of an 'electroacoustic transducer.

Still another object is to provide a transducer of general utility which may be made on general purpose tooling with only a small capital investment required for entry into production. In this connection, an object is to provide a'generalized transducer design which may be varied, at almost no cost in order to cause the transducer to operate over virtually the entire usable range of frequencies without requiring any substantial amount of piece part changes.

In keeping with an aspect of the invention, these and other objects are accomplished by providing a clamped vibratile disk diaphragm, integrally formed in the wall of an attractive housing. Inside the housing, a small disk of piezoelectric material is cemented to the back of the vibratile disk. Suitable electrical'component's, mounted inside the housing, drive the piezoelectric material to vibrate the diaphragm and send'sonic energy outwardly from the front of the vibratile disk, which is on the out-- side surface of the housing.

The nature of the invention will be understood best from a study of the following specification when read in connection with the attached drawings in which:

FIG. 1 is a perspective view of a preferred embodiment of the invention with part of the housing broken away to show the low cost structure;

FIG. 2 is a cross sectional view taken along the line 2 -2 of FIG. 1; and

FIG. 3 is a side elevation profile showing an alternative embodiment of the invention.

FIG. 1 shows an electronic acoustic transducer 10 mounted on a suitable base or housing 11. The transducer 10 comprises a rigid housing structure having a clamped vibratile disk 12 integrally formed in one external wall. Preferably, the outside contours of housing 10 have an attractive shape which is aesthetically pleasing without requiring any further escutcheons or embellishments. Y

The base 11 may have any utilitarian form; it is here shown as a conventional octagonal junction box or electrical receptacle. Since these junction boxes or receptacles are either readily available or are already installed in most buildings, the transducer 10 may be mounted and supported with little or no additional installation work. Of course the invention is not limited to this or any other specific mounting structure. Any suitable base arrangement may be used in lieu of the octagonal junction box or receptacle, here disclosed, and it may serve any convenient function. For example the base 11 is here shown as containing a terminal block 14 for making connections to external equipment.

The details of the transducer 10 may become more apparent from a study of the cut away part of the perspective view of FIG. 1 and of the cross sectional view of FIG. 2.

More particularly, the housing is a somewhat saucer I shaped structure 15 made of any suitable material. For

example, it may be an aluminum (or other metal) die casting. It could also be a rigid plastic molding, or the like, with or without metallic plating thereon. The elements of the saucer are a skirt portion 16 of any convenient design adapted to be mounted on or over the base support member 11. The housing functionally terminates in a disk 12 having a face which is flat on the top or outside of the saucer 10. The inside of the housing is counterbored at 17 to form the inside face or bottom of the disk 12.

As a result of this construction the disk 12 is formed as a thin, flat circular, vibratile diaphragm. The periphery of the disk 12 is clamped by a massive, somewhat cylindrical wall-like element 20. Those who are familiar with the art will readily perceive how the relative thickness of the disk 12 and the surrounding massive clamping area 20 is selected to insure that the vibratile disk 12 operates as a diaphragm. Initially, this selection is made on the basis of the dimensional requirements for operation at the desired operating frequency, but

any errors will be resolved on the side of initially making the disk 12 slightly thicker than desired. I find that usually the counterbored hole 17 is more than three times deeper than the thickness of the disk 12.

A small disk 21 of piezoelectric material, having electrodes on each face surface, is cemented inside the 23 on the bottom of the ceramic disk is connected to a wire 24 for giving an electrical control circuit access to the piezoelectric material. Still other methods of electrode construction, connection, and subdivision are disclosed in U.S. Pat. No. 2,967,957 and incorporated herein by reference.

To protect' the'cerami c material, an insulating plate 25 seals off the chamber formed by .the counterbored hole 17. A hole 26 in plate 25 allows passage of the wire 24 from the piezoelectric material 21 to a second chamber'3l in the housing 15.

A cylindrical or other shaped wall 30 is integrally formed in the housing 1.0 to provide a chamber 31 for receiving electrical components. Of course, separate pieceparts could be used; however, there is generally a cost' advantage if the entire housing is formed as a single, integrally cast or molded housing.

The nature of the electrical components housed inside chamber 31 depends somewhat upon the usage of the transducer. For a particular usage of this preferred embodiment, I show a tuned choke coil 32 and a couslightly thicker than desired. This means that the manufactured resonant frequency is a little higher than the desired resonant frequency.

After final assembly, each transducer is tested, and its resonant frequency is measured. The units are then segregated in groups having predetermined frequency ranges. Each'unit is then machined over the exposed pling transformer 33. The tuned choke and secondary element 21.'When a plastic housing is used, it is necessary to supply a different conductive return path, as by plating either the inside of cavities 17 and 3l.or a small strip of metal extending from cement 22, through the inside of cavity 17 to the pin 34.

Any external electrical equipment is connected to the lead wires 35 and the other winding of transformer 33. These wires 35 pass through a hole 36 in an insulating disk 37 which closes the chamber 31. Any suitable potting compound fills the chamber 31 to protect the electrical elements therein.

The embodiment of FIG. 2 has been drawn with the thought that octagonal junction boxes are convenient supporting structures which are readily available at very low cost. Sometimes, however, the installation considerations may be such that this is not the most convenient mounting means. For example, it may be muchsimplier to bore a hole in a wall and push the transducer into that hole. Hence, the invention contemplates changing the housing shape, as required by any given installation. By way of example, FIG. 3 shows I that'the skirt portions 15, 16 may be removed to leave production tooling in order to meet the operating requirements anticipated by the invention.

In order to reduce production costs, the entire housing and transducer assembly is produced in large quantities, each transducer presumably operating at the same resonant frequency. However, the low cost production introduces a wide variation in manufacturing tolerances. Thus, the disk 12 is deliberately made face of the disk 12 to reduce the thickness of the vibratile diaphragm. It has been found that this may be done by holding the transducer face down on a moving sanding belt. Either the grit or type of sanding belt may be selected for each segregated group of tested units; or, the duration of the sanding operation may be varied to accomplish the desired frequency adjustment.

The objects, advantages, cost reductions, and structural detail described herein will suggest other modifications to those who are skilled in the art. Therefore, the claims are to be construed as covering all equivalents reasonably falling within the scope and the spirit of the invention.

I claim:

1. The method of manufacturing electroacoustic transducer assemblies comprising the steps of:

A. forming a housing structure with a flat surface on an external wall and a recess in an internal wall opposite said flat surface,- the housing material between said flat surface and the bottom of said recess forming a vibratile diaphragm, the housing material surrounding said recess forming a massive clamp around the periphery of said diaphragm,

B. attaching an electrical-to-mechanical transducing element inside said recess to the diaphragm,

C. measuring the resonant frequency of the resulting transducer assembly, and

D. removing material from said flat surface, the amount of removed material being proportioned to the deviation between the measured resonant frequericy and a desired resonant frequency.

2. The method of claim 1 wherein step (A) comprises the further sub-step of initially forming said recess with a depth which is more than three times the thickness of said vibratile diaphragm.

3. The method of claim 2 and the further sub-steps of forming a chamber by means of a second recess in said housing, installing at least some electrical components in said chamber, and connecting said components to drive said transducing element.

4. The method of claim 3 and the further sub-step of filling said chamber with a potting compound.

5. The method of claim 1 and the further sub-step of forming a chamber by means of a second recess in said housing, installing at least some electrical components in said chamber, and connecting said components to drive said transducing element.

6. The .method of claim 5 and the further sub-step of filling said chamber with a potting compound.

7. The method of claim 1 wherein step (A) comprises the further step of forming the external wall of said housing to recede away from said flat surface whereby the material removed in step (D) is removed exclusively from said flat surface and not from the surrounding areas of said externalwall.

8. The method of claim 7 wherein said receding surface is a truncated conical surface and said flat surface is a disk at the apex of said truncated cone.

9. A process for making an electroacoustic transducer comprising the steps of forming:

i A. a'vibratile disk clamped at its periphery to a rigid housing structure, said vibratile disk being an external flat portion free of all upstanding structures protruding above the surface of said disk,

" B. surrounding said disk by a massive somewhat cy- E. bonding one side of said piezoelectric disk to the inside'surface of said recessed cavity opposite said external flat portion,

F. attaching electrical conductors to the electrode on the other side of said disk, whereby the entire disk experiences the same instantaneous electrical stresses,

G. driving said transducer by applying a driving signal to said conductor, I

H. measuring the frequency of said transducer while said signal is applied to said conductor, and

removing material from said external flat portion,-

the amount of removed material being proportioned to the deviation between the measured frequency and a desired predetermined frequency. k 

1. The method of manufacturing electroacoustic transducer assemblies comprising the steps of: A. forming a housing structure with a flat surface on an external wall and a recess in an internal wall opposite said flat surface, the housing material between said flat surface and the bottom of said recess forming a vibratile diaphragm, the housing material surrounding said recess forming a massive clamp around the periphery of said diaphragm, B. attaching an electrical-to-mechanical transducing element inside said recess to the diaphragm, C. measuring the resonant frequency of the resulting transducer assembly, and D. removing material from said flat surface, the amount of removed material being proportioned to the deviation between the measured resonant frequency and a desired resonant frequency.
 2. The method of claim 1 wherein step (A) comprises the further sub-step of initially forming said recess with a depth which is more than three times the thickness of said vibratile diaphragm.
 3. The method of claim 2 and the further sub-steps of forming a chamber by means of a second recess in said housing, installing at least some electrical components in said chamber, and connecting said components to drive said transducing element.
 4. The method of claim 3 and the further sub-step of filling said chamber with a potting compound.
 5. The method of claim 1 and the further sub-step of forming a chamber by means of a second recess in said housing, installing at least some electrical components in said chamber, and connecting said components to drive said transducing element.
 6. The method of claim 5 and the further sub-step of filling said chamber with a potting compound.
 7. The method of claim 1 wherein step (A) comprises the further step of forming the external wall of said housing to recede away from said flat surface whereby the material removed in step (D) is removed exclusively from said flat surface and not from the surrounding areas of said external wall.
 8. The method of claim 7 wherein said receding surface is a truncated conical surface and said flat surface is a disk at the apex of said truncated cone.
 9. A process for making an electroacoustic transducer comprising the steps of forming: A. a vibratile disk clamped at its periphery to a rigid housing structure, said vibratile disk being an external flat portion free of all upstanding structures protruding above the surface of said disk, B. surrounding said disk by a massive somewhat cylindrical wall-like member, said massive wall being defined by a recessed circular cavity in the internal wall surface of said housing, the bottom of said cavity being opposite said external flat portion, C. forming a single disk of piezoelectric material having a diameter that is substantially smaller than the diameter of said recessed cavity, D. forming an electrode surface on each opposite face side of said piezoelectric material, said electrode surface covering substantially the entire surface of said single disk of piezoelectric material, E. bonding one side of said piezoelectric disk to the inside surface of said recessed cavity opposite saiD external flat portion, F. attaching electrical conductors to the electrode on the other side of said disk, whereby the entire disk experiences the same instantaneous electrical stresses, G. driving said transducer by applying a driving signal to said conductor, H. measuring the frequency of said transducer while said signal is applied to said conductor, and I. removing material from said external flat portion, the amount of removed material being proportioned to the deviation between the measured frequency and a desired predetermined frequency. 